A typical use of adhesives is for assembling articles made from different individual component parts. For example, an article may be composed of a core material that is sandwiched between a rigid outer material. The most common core materials are those that are porous and lightweight. Examples of such core materials include polystyrene and polyurethane foams and papers and metals in the form of a honeycomb mesh. Common rigid outer materials include metals such as cold rolled steel, galvanized steel, phosphate treated steel, stainless steel, and aluminum.
In a typical manufacturing process, an adhesive is applied to one or both surfaces of a core material and a rigid outer material and then pressed together under pressure. In addition, the bonded assembly may be required to meet certain performance tests. For example, the bonded assembly may be required to provide a substantial green tensile strength and a final tensile strength that is even higher.
In order to attempt to meet these requirements, adhesive formulators have provided water-based contact adhesives. In general, water-based contact adhesives are easy to use and are typically inexpensive. For example, water-based contact adhesives can be applied to substrates by spraying or roll coating. However, both surfaces of the substrates to be bonded must be coated to form an adhesive bond which limits production flexibility. Additionally, the surfaces must be joined in a relatively short period of time in order to form a bond. Moreover, once the parts have been joined together, the parts cannot easily be repositioned. Further, the coating of both substrates to be bonded, the types of substrates to be bonded, and the method of application of the contact adhesive can result in waste of adhesive. Finally, currently available water-based contact adhesives have not demonstrated adequate bond strength and temperature resistance for some applications.
Formulators have also provided water-based adhesives containing polyurethane dispersions. These adhesive compositions are typically applied to a substrate to be bonded, dried, and then bonded using heat and pressure. Such dispersions that contain dispersed polyurethanes that are primarily amorphous only possess one glass transition temperature (Tg) generally below 0.degree. C. Such adhesives are not generally suitable for forming a thermal bond as their storage modulus (G') over a measured temperature range tends to be relatively flat, necessitating higher temperatures to form a bond. Water dispersed adhesives containing carboxylated and sulfonated crystallizing polyester urethanes are known. Adhesives containing primarily crystallizing polyester urethanes may be relatively stiff or inflexible and typically have short open times, have limitations on drying temperatures, and have inadequate adhesion to metallic surfaces. Typically, such adhesive compositions are combined with an isocyanate to increase their thermal resistance.